Ever since Stephen Sondheim reached the four-score mark three years ago, the spotlight has shifted from his work to the legend himself. There have been birthday galas, musical tributes, interviews galore and books in which the master reveals the secrets of his songwriting sorcery.

Now there's "Six by Sondheim," an HBO documentary airing Monday that's directed by Sondheim's frequent collaborator James Lapine, the librettist and director without whom there'd be no "Sunday in the Park With George," "Into the Woods" or "Passion."

The film, which has a limited engagement at Pasadena's Laemmle Playhouse 7 starting Friday, doesn't fill in any major gaps in our knowledge of the relationship between Sondheim's life and art. But it does stylishly retread this fascinating ground with a rising emotional thrum that renders absurd the notion that Sondheim is all neurotic cleverness and no heart.

With so much golden musical material, an organizing structure is needed, and Lapine, who conceived and directed the 2010 Broadway salute "Sondheim on Sondheim," finds one in six classic songs: "Something's Coming" ("West Side Story"), "Opening Doors" ("Merrily We Roll Along"), "Send in the Clowns" ("A Little Night Music"), "I'm Still Here" ("Follies"), "Being Alive" ("Company") and "Sunday" ("Sunday in the Park With George").

Weaving Sondheim history and personal refection in and around these numbers, "Six by Sondheim" proceeds as though piecing together a jigsaw puzzle portrait of the man who redefined the American musical in the second half of the 20th century.

By splicing interviews of the eager twentysomething lyricist-composer with the restless (and somewhat prickly) mid-career explorer and the grizzled elder statesman, the film provides a guided tour of Sondheim's artistic consciousness along with his changing hairstyles and beards.

Few artists are as articulate about their own creative process. But what's most striking in this retelling is the emotional debt Sondheim acknowledges to Oscar Hammerstein II, his surrogate father.

Sondheim considers Hammerstein an "experimental playwright" and vigorously defends him against those who refuse to see just how far he extended the boundaries of the Broadway musical. His entire generation, he asserts, was instructed by Hammerstein's mastery of resonant simplicity, the art of conveying complex feelings in honest and direct language and imagery.

Although their styles are markedly different, Sondheim credits Hammerstein with instilling in him the value of writing for oneself and not for commercial gain. But it's the personal relationship that causes Sondheim to well up.

As a young man caught in the crossfire of his parents' nasty divorce, Sondheim found sanctuary in the Hammerstein household. Looking back as an older man, he speculates that he might not be alive were it not for Hammerstein's benevolent interest in him. No one taught him more about the life-changing impact of teaching, which he calls "the sacred profession."

Shooting down canards is one of Sondheim's great hobbies, and a great number of clips are devoted to him dismantling false assumptions about his work. Of course his music doesn't need to be defended against charges that it lacks a heart or isn't sufficiently hummable. All it needs is to be listened to.

Tantalizing archival footage of Ethel Merman in "Gypsy" and Bernadette Peters in "Sunday in the Park With George" reminds us of just how many superstar careers have fatefully intersected with Sondheim's. He refers to himself as a Broadway baby and his theatrical legacy is inseparable from all those performers whose legends were sealed in his shows.

Showstoppers are traced back to the singers who inspired them. "I'm Still Here" borrowed from the roller-coaster life of Joan Crawford to give Yvonne De Carlo a number worthy of her celebrity. The short line phrases of "Send in the Clowns" was designed to accommodate Glynis Johns' breathy vocal style, though a delightful YouTube montage of this unexpected Sondheim hit demonstrates just how flexible this lyrically baffling number can be.

A sprightly new filmed version of "Opening Doors" with Darren Criss, America Ferrera and Jeremy Jordan entrusts to the next generation this singing portrait of young artists in their salad days. Sondheim refers to this as his most explicitly autobiographical number, explaining that his usual songwriting practice is to inhabit the situation of his characters the way an actor inhabits a role.

There are other contemporary cracks at classics — a "Send in the Clowns" with Audra McDonald and Will Swenson and an outré version of "I'm Still Here" with Jarvis Cocker of the rock band Pulp that's directed by Todd Haynes. But it's the vintage performances that steal the film.

In an excerpt from the superb 1970 D.A. Pennebaker documentary "Company: Original Cast Album," Dean Jones delivers a rendition of "Being Alive" that beats any I've heard live or on recording. And what a thrill to see Larry Kert, the young star of the original Broadway production of "West Side Story," knock "Something's Coming," with all its fleet baseball imagery, out of the park.

Still hard at work at a craft that he says requires only more courage with age, Sondheim hasn't always received the appreciation he deserves. Those who radically advance their art forms rarely do. So it's gratifying to see him recognized as one of the immortals while he's still around to enjoy it.